Typically, gas turbine engine fuel controls utilize positive displacement pumps to supply fuel at the necessary high pressures. However, positive displacement pumps require close operating clearances and contain parts with highly stressed metal-to-metal contacts which wear rapidly in low lubricity fuel. In addition, the performance of positive displacement pumps may be adversely affected by the presence of contaminants.
While high speed centrifugal pumps are relatively insensitive to contaminants and are capable of generating the fuel pressures required for engine operation in their normal speed range, such pumps will not generate sufficient fuel pressure at engine light-off or cranking speed (which may typically be 10% to 20% of maximum engine speed). In order to overcome the aforementioned drawback, centrifugal pumps have been combined with positive displacement pumps to form pumps capable of extending the lower range of operation. An example of such a combination pump is shown in U.S. Pat. No. 3,851,998. The major undesirable characteristic of a combination pump is that it must be relatively complex.
U.S. Pat. No. 3,576,375 offers a solution to the previously discussed problems by providing a fuel pump with two impeller elements, one for starting and one for normal operation. While the latter described arrangement may successfully pump fuel at the necessary pressures, it requires that the two impeller elements be mounted in separate chambers and interconnected by means such as a common drive shaft. Moreover, the chamber in which the starting impeller is located must be drained of fluid to eliminate fluid resistance during normal operation or alternatively, some form of clutch mechanism must be employed.